In processes for manufacturing paper and in processes where cellulose-containing fiber pulp is processed for other purposes there are different steps where the pulp suspension, which comprises cellulose-containing material, such as fibers, mixed with a liquid, such as a liquor, is processed under pressure in different kinds of pressurized containers. An example of such a step is the cooking of pulp, where wood chips are fed into a pressure-tight container of a pulp digester and are processed or treated at a certain temperature and under a certain pressure. European Patent No. 420,791 discloses a process and a mechanism for cooking pulp and discharging pulp digesters. Another example of a step in which pulp suspensions are processed under pressure is screening, where the pulp suspension is fed into a pressure-tight container, and under pressure is screened to separate the reject, comprising material with too large dimensions, from the accept, which is transported further in the process for subsequent processing steps in the process. U.S. Pat. No. 6,186,333 discloses a method and an apparatus for screening pulp suspensions.
When cooking in pressurized containers, as mentioned above, valves are used at the outlet of the container, which valves are closed during the process or treatment to maintain the pressure in the container and thereafter are opened when discharging pulp suspension from the container. In the screening disclosed in U.S. Pat. No. 6,186,333, there are valves in the outlet channels for both the reject and the accept. However, there are problems when using valves at the outlets of pressurized containers.
When valves are used in pressurized screening devices for outlets with small pulp flows, the cross-section of the passage where the valve is provided must be small in order to maintain a sufficiently high pressure in the container. This is, for example, the case for the reject outlet of pressurized screening devices where the reject flow often is small. The reduced cross-section area in these passages produce a flow resistance, and the result is that clogging easily occurs, which results in operational disturbances. A solution to this problem is to add additional dilution fluid prior to the valve. However, this is not desirable as it negatively affects the liquid balance. In European Patent No. 280,234, a solution to the problem of flow resistance in a valve used in a line for processing and controlling pulp suspensions of a high pulp consistency is disclosed. Here, the pulp suspension is fluidized just prior to the valve by a rotating rotor which is provided inside the valve body. The purpose of this rotating rotor is also to efficiently mix added chemicals with the pulp suspension, which is effected by adding chemicals in connection with the rotor's fluidization of the pulp suspension.
Problems also arise when valves are used in connection with pulp digesters at the outlet for discharging the pressurized container of the pulp digester. When the valve in the outlet of the container is opened after the cooking is finished, the pulp suspension is blown out of the container in an uncontrolled manner by the pressure produced in the container, which produces unwanted channeling of the pulp suspension in the container, which, inter alia, results in pulp remaining along the inner walls of the container and getting stuck. This pulp must then be removed, e.g. by pressurized washing, prior to the next cooking, which results in stops in the production. European Patent No. 420,791 discloses a solution to this problem and additional problems resulting from such blow-out of pulp suspensions, where the valve in the outlet of the container is cyclically opened and closed during the discharge, which is suggested to prevent the channeling in the pulp suspension.
However, the solutions of European Patent No. 420,791 and European Patent No. 280,234 do not provide a sufficiently efficient or controlled flow of pulp suspension from a pressurized container.
One object of the present invention is thus to provide a more efficient control of a flow of pulp suspension from a pressurized container in relation to prior art.